one-piece has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are two-piece, three-piece.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(of clothing) consisting of or fashioned in a single whole piece
“a one-piece garment”
Synonyms, antonyms and the broader and narrower relations for one-piece come from Princeton University’s WordNet. Full sources and licences.